It is common practice, and has been for many years, to pelletize solid, particulate dusty materials for storing, handling and transporting the materials. Dusty materials, referred to herein, include solid particulate matter which is reduced to minute portions as in a fine powder, and normally dusty materials refer to those powders which generate dust when they are transferred from one container to another. Normally dusty materials are exemplified by finely ground silica less than 325 Tyler mesh in size, finely ground pigments, flour, talc, clay, particulate electrodeposited metals and metal oxides, carbon black, various forms of activated carbon, and the like. In general, all finely divided powders with a significant superfines content, having particles less than about 37 microns in size, which exhibit a notorious proclivity to dust are usually referred to in industry as normally dusty materials or normally high-dusting materials. It is to all dusty materials that the article of manufacture of this invention is directed.
Pelletizing of dusty material is commonly effected from a fluid paste or a slurry incorporating a binder, in what is referred to as a wet process. Typically, a paste of a liquid binder and particulate material is extruded through a die to yield an extrudate, or strands of a thick paste are formed on a grooved drum. The extrudate or strands are thereafter chopped to provide dense, damp pellets of predetermined size. The damp pellets are subsequently dried into relatively small shapes, not necessarily regular, from about 1/8 inch to about 1/2 inch in diameter. The dried pellets, because of the cohesive effect of the binder, are relatively strong and can withstand considerable pressure. For example, dense, dried pellets at the bottom of a 55-gallon drum will normally retain their shapes with only minor attrition. When dense pellets break, due to mechanical shock, they tend to shatter.
The agglomerate of the instant invention may be formed in the dry state, without the benefit of a liquid binder. Where, for convenience in processing, a liquid dispersing medium is used to effect a homogenous dispersion of fibrillatable PTFE in a normally dusty material prior to separating the liquid therefrom, and a small amount of residual liquid is left in the mixture, the mixture may nevertheless be worked effectively; the presence of residual liquid is unnecessary during the working of the mixture to form the novel agglomerate.
Certain dusty materials, such as carbon black, lend themselves to agglomeration not only by a wet process referred to hereinabove, but also by a dry process in which, despite lack of a binder, a pellet is formed in which particle-to-particle adhesion is obtained by virtue of the relatively high bulk density of the pellet, usualy from 1.5 to 5 times numerically greater than that of the dusty material in its normal quiescent state. Other materials, particularly powder metals and organic powders, are commonly tabletted in the dry state, often with a solid binder. A typical tablet, formed in the dry state, is an aspirin tablet. Pellets formed by the dry process have the obvious advantage of being free from contaminating binders and at the same time, because of their greater density, provide a shrunk volume which reduces shipping charges. PTFE resin is a known lubricant for tabletting a variety of particulate materials.
Processes for the formation of densified green articles with particle-to-particle adhesion, which articles are subsequently sintered or heat-treated, are unrelated to the process of this invention or to the novel agglomerate formed thereby. The novel agglomerate is not sintered and no attempt is made to invest it with that degree of structural integrity characteristic of pellets adapted to withstand handling and to maintain their shape during transportation by virtue of their density. Prior art agglomerates have been described as fragile and friable, though dense and rugged. The criterion of friability was generally set by the ability of the pellet to be incorporated into a mixture in heavy equipment, such as a Banbury mill, and the self-evident need for ruggedness was determined by the desirability of presenting the pellet to the mill without permitting it to disintegrate. In contrast, the need to which the instant process is directed is primarily to provide a dustless agglomerate substantially free of particle-to-particle adhesion, with essentially no structural integrity and which is at the same time both friable and pliable at ambient temperature and pressure conditions, as will be described more fully hereinfter. It will be recognized that the formation of the week agglomerate of ths invention has little in common with the formation of an agglomerate from a pasty, sticky or gummy resinous mass, of from a solid particulate mixture having a relatively large amount, greater than 1 percent by weight, of PTFE.
In the sense that the finely divided normally dusty powder is held together in an agglomerate without particle-to-particle adhesion, the binder in the instant invention is provided by a matrix of microscopic and submicroscopic fibers of fibrillatable fine powder PTFE resin which is a homopolymer of tetrafluoroethylene in the form of spheroidal porous aggregates with an average diameter of about 450 microns. Commonly available granular molding powder of PTFE resin is not fibrillatable and is ineffective to form the desired matrix.
U.S. Pat. No. 3,005,795 discloses synthetic polymeric resin mixtures of normally rigid thermoplastic polymers which contain small amounts of fine, fibrous particles of fibrillatable polytetrafluoroethylene resin incorporated by milling the molten resin with from 0.05 to 20 percent by weight of PTFE. Polymers modified with PTFE exhibit higher melt elasticity than the unmodified polymers and are more suitable in forming operations, such as wire coating, extrusion and thermoforming.
U.S. Pat. No. 3,326,731 discloses a plastic explosive composition which utilizes from 5 to 25 percent by weight of PTFE resin. The plastic explosive composition formed is pliable, surprisingly strong and can be inelastically extended 50 percent or more.
U.S. Pat. Nos. 3,455,749 and 3,466,204 discloses a plastic explosive composition and a process for making it in which PTFE in the range from 1-5 percent by weight is usually sufficient to give compositions which can be formed in coherent plastic state under very light compression and which have a consistency somewhat resembling the confection known as marshmellow. Thses mucilaginous, soft, creamy and pliable compositions are not friable.
British Pat. No. 891,537 discloses the use of PTFE as a binding agent, by itself or in combination with other binding agents. The binding agents may be added at any convenient stage, as for example, to an aqueous slurry of a particulate powder to form an aqueous suspension from which most of the water is removed to form a paste. The paste so obtained may be shaped into pellets, granules and other agglomerated forms in any manner known in the art, for example, by extrusion through a die, or by forming on a grooved drum. The proportion of binding agent to be employed may be in the range from 0.05 to 12 percent of the weight of the powdered organic chemical substance which is pelleted.
The physical properties of known compositions with PTFE contents in the range 1-5 percent by weight are surprisingly different from the properties of an agglomerate of normally dusty material which contains less than 1 percent by weight PTFE, and preferably from about 0.02 to about 0.75 percent by weight, based on total solids. For example, a relatively large agglomerate is weak, pliable and easily deformed under light pressure between a person's fingers; it has essentially no stability of shape where a multiplicity of agglomerates are stored in a large receptacle, such as a drum, bin or hopper, or are transmitted in a shipping container.
Again, known compositions containing in excess of 5 precent by weight PTFE have physical properties, for example, surprising strength and inelastic extensibility greater than 50 percent, which are quite unlike the combination of weak, friable and characteristically tearable properties of the agglomerate of this invention. Significantly, this combination of properties, heretofore deemed undesirable in a transportable agglomerate, imbue the agglomerate with an essentially total freedom from dusting and exceptional dispersibility in other materials.